1. Field of the Invention
The present invention relates to an electroluminescent device using an organic compound, and more particularly, to an organic electroluminescent device from which light is emitted by applying an electric field on a thin film made of an organic compound.
2. Related Background Art
An organic electroluminescent device is a device that includes a thin film made of a fluorescent organic compound between an anode and a cathode, generates an exciton from the fluorescent organic compound by injection of an electron and a hole from each electrode, and utilizes light to be radiated when the exciton returns to the ground state.
The study conducted by Eastman Kodak Company in 1987 (Non-Patent Document 1) reported light emission on the order of 1,000 cd/m2 at an applied voltage of about 10 V, from a device having a separated-function type two-layer structure where an anode is made of ITO and a cathode is made of magnesium-silver alloy, an aluminum quinolinol complex is used as each of an electron-transporting material and a light-emitting material, and also a triphenylamine derivative is used as a hole-transporting material. In this case, related patent documents include Patent Documents 1 to 3.
In addition, light emission at spectra ranging from ultraviolet through infra-red is allowed by changing the type of the fluorescent organic compound. Recently, therefore, various compounds have been studied extensively and described in, for example, Patent Documents 4 to 11.
Furthermore, in addition to the organic electroluminescent devices using low molecular weight materials as described above, an organic electroluminescent device using a conjugated polymer has been reported from the group of the Cambridge University (Non-Patent Document 2). This report confirmed light emission from a monolayer by film formation with polyphenylene vinylene (PPV) in a coating system. Patents related to the organic electroluminescent device using the conjugated polymer include Patent Documents 12 to 16.
Recent advances in technology concerning organic electroluminescent devices are remarkable and the characteristics thereof allow the formation of thin and lightweight electroluminescent devices having high luminance at a low applied voltage, a variety of emission wavelengths, and high-speed response, suggesting the possibility to extensive uses.
However, many problems still remain to be solved in terms of durability, such as changes over time by prolonged use, or degradation with atmospheric gasses including oxygen, humidity, or the like. Considering applications to full-color displays and so on, under present circumstances, high-luminance optical output and high-efficiency emission have been demanded.
As examples of the materials for organic electroluminescent devices containing anthracene rings, phenyl anthracene derivatives are disclosed in Patent Document 17. In particular, when used as a blue light-emitting material or an electron-injection transporting material, the derivatives are assumed to allow the formation of a good organic film because of its low crystallinity. However, the light-emitting efficiency and useful life thereof are insufficient in practical use.
As other examples, an aminoanthracene derivative and a diaminoanthracene derivative have been disclosed in Patent Documents 18 and 19, respectively. In the documents, those materials are assumed to emit green light when used as light-emitting materials. However, devices prepared from those materials have insufficient light-emitting efficiencies and their useful lives are still insufficient in practical use.
As another example, Patent Document 20 discloses a device using a specific bianthryl compound as a light-emitting material, which is assumed to attain light emission with high luminance. However, there is no description about light-emitting efficiency and useful life.
As still another example, Patent Document 21 discloses a device using a specific anthracene compound having an olefin portion as a light-emitting material, which is assumed to obtain light emission from yellow to red. However, the device has insufficient light-emitting efficiency in practical use.
Furthermore, as yet another example, Patent Document 22 discloses a device that contains an anthracene derivative having a specific structure, an electron-transporting compound, and another fluorescent compound in a light-emitting medium layer. It is assumed to obtain a red light-emitting device with improved reliability. However, the device has insufficient light-emitting efficiency in practical use. In addition, it is difficult to obtain blue light emission because of its device configuration.
[Patent Documents]
1. U.S. Pat. No. 4,539,507 B
2. U.S. Pat. No. 4,720,432 B
3. U.S. Pat. No. 4,885,211 B
4. U.S. Pat. No. 5,151,629 B
5. U.S. Pat. No. 5,409,783 B
6. U.S. Pat. No. 5,382,477 B
7. JP H02-247278 A
8. JP H03-255190 A
9. JP H05-202356 A
10. JP H09-202878 A
11. JP H09-227576 A
12. U.S. Pat. No. 5,247,190 B
13. U.S. Pat. No. 5,514,878 B
14. U.S. Pat. No. 5,672,678 B
15. JP H04-145192 A
16. JP H05-247460 A
17. JP H08-012600 A
18. JP H09-157643 A
19. JP H10-072579 A
20. JP 3008897 B
21. JP H11-008068 A
22. JP 2001-284050 A
[Non-Patent Documents]
1. Appl. Phys. Lett., 51, 913 (1987)
2. Nature, 347, 539 (1990)